Even the simplest of organisms appear to have internal clocks of some type. In order to understand this phenomenon, researchers have focused much attention on understanding the internal clock of the Neurospora crassa, a kind of mold that produces spores of a bright orange color. Jonathan Arnold, a researcher at the University of Georgia, Athens, and his colleagues believe they have found how the mold's 'clock' operates. At bottom, the biological talent for keeping time involves some type of regular, chemical oscillation, or a molecular equivalent of a swinging pendulum. The cells that perform the timekeeping must first change their chemistry in a predictable way and then return to their original state in an equally predictable way. Theoretically, this does not require much, but finding out how the Neurospora crassa does this has not been easy. Arnold decided to use an historical approach to the problem, and while he has not achieved a final, definitive model, he has been able to assign probabilities to the likelihood of each element in a model to be right. Arnold began with a small group of genes, a list of 25 chemical reactions known to happen and thought to be relevant, and a few 'snapshots' of how much of various proteins and other molecules could be found in the mold cells at any one time. He then put these values into the computer and generated all possible genetic circuits that could account for these clues.